Ethylenically unsaturated compounds, such as acrylate derivatives, can be polymerized by exposure to radiation, typically ultraviolet light, in the presence of a photoinitiating system. Typically, the photoinitiating system includes (1) a compound capable of initiating polymerization of the ethylenically unsaturated compound upon exposure to radiation (a "photoinitiator") and optionally (2) a coinitiator or synergist, that is, a molecule which serves as a hydrogen atom donor. The coinitiators or synergists are typically alcohols, tertiary amines or ethers which have labile hydrogens attached to a carbon adjacent to a heteroatom.
Numerous photoinitiators with varying structures are commercially available for use in different systems. However, nearly all commercially available radiation curing processes require an initiator incorporated into the formulation, a large percent of which is not consumed. The use of conventional photoinitiators typically results in the production of small molecule photo-byproducts. The presence of the residual photo-active compounds and extractables can result degradation of the physical properties of the article, such as decreased light fastness, discoloration, and lower resistance to oxidative degradation. In addition, the residual photoinitiator can be extracted or leach out of the cured article or migrate to the surface of the article, which is undesirable in many applications.
Increasingly stringent environmental protection legislation has prompted the exploration and use of formulations which contain little or no volatile organic compounds (typically solvents). Thus the use of formulations with close to 100% reactive component is of great interest.
Aliphatic maleimides have been proposed as useful photoinitiators for the photopolymerization of acrylate systems via hydrogen abstraction mechanisms. However, prior attempts to provide aromatic maleimide compounds which photoinitiate via hydrogen abstraction mechanisms have been entirely unsuccessful.